The present invention relates to a current switching type logic circuit, and more particularly to a logic circuit having a function to control temperature characteristics of a logic voltage produced therefrom by adjusting a resistor provided in a shunt circuit including a diode, connected to a load resistor. Specifically, the present invention is concerned with a current switching type logic circuit wherein a function to control temperature characteristics of logic voltage is provided, thereby enabling controlling of temperature characteristics of and output low level voltage produced from a level shift circuit provided at the output stage.
For controlling temperature dependency over a swingable range of a logic voltage in the conventional current switching type logic cicuits, there has been employed a method of controlling a voltage source for applying a voltage to the base of a transistor constituting a constant current source.
FIG. 1 is a circuit diagram showing an example of a conventional current switching type logic circuit such as disclosed in "THE ECL HANDBOOK SECTION SELECTOR", 1974, July, p 2-7, FIGS. 2-9.
The current switching type logic circuit comprises a current switching circuit 10 and a level shift circuit 20 coupled to the output stage thereof. The current switching circuit 10 includes a differential amplifier circuit 12 and a constant current source 14. The differential amplifier circuit 12 comprises a pair of npn transistors Q.sub.1 and Q.sub.2 of which emitters are commonly connected to a node N.sub.0 The transistor Q.sub.1 has a collector connected to a node N.sub.1 and a base to which an input signal V.sub.IN is applied. Likewise, the transistor Q.sub.2 has a collector connected to a node N.sub.2 and a base to which a reference voltage V.sub.ref is applied. The constant current source 14 includes an npn transistor Q.sub.3 having a collector connected to the node N.sub.0, a base to which a voltage V.sub.CS from a voltage source for a constant current control is applied, and an emitter connected to a power source V.sub.EE through a resistor R.sub.3. The current switching circuit 10 is further provided with two load resistors R.sub.1 and R.sub.2. One end of each of the resistors R.sub.1 and R.sub.2 is connected to the respective nodes N.sub.1 and N.sub.2, and the other end is connected to a power source V.sub.CC. Thus, in response to the input signal V.sub.IN and the reference voltage V.sub.ref, the current switching circuit 10 effects a switching operation to produce a logic amplitude V.sub.l swingable within a predeterminded range as a voltage appearing across the load resistor R.sub.2.
Further, the level shift circuit 20 is configured as an emitter follower circuit comprising an npn transistor Q.sub.4. The transistor Q.sub.4 has a collector connected to a power source V.sub.CCA, a base connected to the node N.sub.2 of the current switching circuit 10, and an emitter connected to an output node N.sub.3 and to a power source V.sub.T through a terminating resistor R.sub.T. Thus, the level shift circuit 20 is operative to level-shift the logic voltage from the current switching circuit 10 to produce an output voltage V.sub.OUT swingable between an output high level voltage V.sub.OH and an output low level voltage V.sub.OL.
In the circuit thus configured, assuming that a voltage between the base and the emitter of the transistor Q.sub.3 is represented by V.sub.BE(Q.sbsb.3 .sub.), the logic amplitude V.sub.l is expressed as follows: EQU V.sub.l =(V.sub.CS -V.sub.BE(Q.sbsb.3.sub.)).times.(R.sub.2 /R.sub.3)(1)
The temperature characteristic of the logic voltage V.sub.l is determined depending on temperature change characteristics of the voltages V.sub.BE(Q.sbsb.3.sub.) and V.sub.CS , which generally decrease according as temperature increases.
Because the voltage V.sub.BE(Q.sbsb.3.sub.) is physically determined, it is required to control the temperature characteristic of V.sub.l by the voltage V.sub.CS. For this reason, the drawback with the conventionall circuit is that it is impossible to obtain logic amplitude V.sub.l or output low level voltage V.sub.OL having different temperature characteristics by using a voltage V.sub.CS from the same voltage source for constant current control.